The present invention relates to apparatus for making graphic products on sheet material, and more particularly, to such an apparatus having a replaceable cassette carrying an ink web bearing printing ink, and a take-up motor for driving the ink web with the sheet material between a platen and a print head for printing graphic images on the sheet material.
There are several commercially-available systems today that employ thermal print heads to transfer ink from an ink web to a strip of sheet material to produce graphic products with multicolored or enhanced graphic images for signs and like displays. One such commercially-successful system is manufactured and sold by Gerber Scientific Products, Inc. of Manchester, Conn. under the trademark GERBER EDGES. The GERBER EDGE(trademark) is typically used to print graphics for signs or like displays, wherein multicolored or enhanced graphic images are typically printed on a vinyl or like polymeric sheet, and the sheet is cut along the periphery of the graphic images to create a sign or like display. The system uses a thermal print head to print the graphic images on the sheet, and a cutter to cut the sheet along a peripheral edge surrounding the graphic images. The print head and the cutter are controlled by a microprocessor having a common data base so that the printed images and the cut edges correspond positionally in the final graphic product.
A roller platen carrying the sheet material is mounted below the print head, and a removable cassette carrying a donor web bearing transfer ink is mounted adjacent to the print head so that the donor web is interposed between the print head and the sheet material. The print head presses the donor web against the sheet material and the heating elements of the print head are selectively energized to transfer ink from the web to the sheet in accordance with commands from the microprocessor to create graphic images on the sheet. Each cassette carries a donor web bearing a single color of transfer ink, and the cassettes are interchanged to create multicolored images, different shades and/or colors. The roller platen and sheet material are slewed back and forth during printing operations to apply the different color inks.
The GERBER EDGE(trademark) system described above is disclosed in U.S. Pat. No. 5,537,135, entitled xe2x80x9cMethod And Apparatus For Making A Graphic Productxe2x80x9d, which is assigned to the Assignee of the present invention, and is hereby expressly incorporated by reference as part of the present disclosure.
A typical ink web used in such systems is comprised of a resin and/or wax layer containing the transfer ink, a release layer superimposed over the resin/wax layer, a carrier layer superimposed over the release layer, and a back coat superimposed over the carrier layer to provide a low-friction surface for engaging the print head. When the heating elements of the thermal print head are energized, the portions of the resin layer contiguous to the print head undergo transformation from (i) a solid state, to (ii) a semifluid or viscous state, and at the highest temperatures, to (iii) a less viscous, liquid state. Then, as the heating elements are de-energized and upon passage of the ink web and sheet material beyond the print head, the heated portions cool down and return from the liquid, semifluid or viscous states to the solid state, as they approach ambient temperature.
During these changes in physical states, the coefficients of friction and thus the forces transmitted between the sheet material, ink web and print head vary, which leads to variations in the surface velocity of the sheet material, and can in turn cause sagging or like deformation in the sheet as it passes beneath the print head. Typically, the longer the print head (i.e., the dimension of the print head in the axial direction of the platen), the greater are the variations in the forces applied to the sheet material. Because the vinyl and like polymeric sheets are flexible, the increase in the forces transmitted between the sheet and print head on each cool-down cycle can cause a lag or positional error between the portions of the sheet contiguous to the print head and other areas, such as the marginal portions of the sheet engaging the sprockets. These variations in the sheet material velocity and positional errors lead to fluctuations in image intensity and, concomitantly, a degradation in print quality.
In a typical commercially-available system the cassette has a supply spool carrying the ink web and a take-up spool for receiving the ink web upon passage beneath the print head. A take-up motor is coupled through an electromagnetic slip clutch to the take-up spool to tension and wind the ink web onto the take-up spool and to assist in driving the sheet material engaged with the ink web beneath the print head. It is perceived as desirable to provide as much ink web as possible in a cassette in order to increase the cassette""s operational life and thereby reduce the frequency at which the cassettes must be replaced. As a result, however, the transfer of ink web from the supply spool to the take-up spool typically causes a significant change in the overall diameter of the take-up spool from the time a cassette is first installed to the time a cassette is near depletion, sometimes on the order of at least about 20 to 25%. Because the tension applied to the ink web is a function of the overall diameter of the take-up spool with web, there is a corresponding change in the tension applied to the ink web as it is wound onto the take-up spool.
Accordingly, the ink web tension may substantially vary from the time a relatively long printing operation is initiated to the time it is completed, or when cassettes at different stages of use are interchanged to apply different colors during printing operations or to replace a depleted cassette. In addition, the electromagnetic clutches used in current systems have a tendency to wear down during the operational life of a system and thereby fail to impart a constant torque to the take-up spool during printing operations, further compounding the variations in, and lack of control over ink web tension. These variations in web tension create corresponding variations in the sheet material and ink web velocity, which in turn lead to further errors in the registration of the print head with the sheet material and compound any degradation in print quality as described above.
It is an object of the present invention to overcome the drawbacks and disadvantages associated with such variations in ink web tension in apparatus for printing graphic images on sheet material.
The present invention is directed to an apparatus for printing graphic images on sheet material comprising a print head, such as a thermal print head having a linear array of heating elements, which is pressed into engagement with an ink web overlying the sheet material on a platen. The platen may be, for example, a roller platen which is rotatably driven to in turn drive the sheet material with the ink web relative to the print head. A removable cassette having a predetermined length L of ink web bearing a printing ink is mounted adjacent to the print head with the ink web interposed between the sheet material and the print head for printing the graphic images on the sheet. A supply spool carrying the ink web is rotatably mounted within the cassette, and a take-up spool is also rotatably mounted within the cassette for receiving the ink web from the supply spool upon passage between the platen and print head. The take-up spool defines a first overall diameter D1 without receiving the ink web from the supply spool, and a greater second overall diameter D2 upon receiving the predetermined length L of ink web, wherein the second overall diameter D2 is within approximately 10% of the first overall diameter D1. The apparatus further comprises means for applying a constant torque to the take-up spool, preferably a spring-wrapped clutch, to thereby maintain a substantially constant tension within the ink web during printing operations.
One advantage of the apparatus of the present invention is that the first overall diameter D1 of the take-up spool and the predetermined length L of the ink web in the cassette are selected so that the overall diameter of the take-up spool cannot vary by more than 10% upon receiving the ink web. This, in combination with the constant-torque clutch, preferably a spring-wrapped clutch, maintains a substantially constant tension within the ink web during printing operations throughout the operational life of the apparatus, and thereby maintains precise registration between the print head and the sheet material in order to accurately print high-quality graphic images. As also described in detail below, the apparatus of the invention preferably employs a platen drive for transmitting a substantially constant force per unit width,to the sheet material to drive the sheet material and ink web at a substantially constant velocity across their width, along with encoded sprockets or like registration means engaging the marginal portions of the sheet material to thereby further facilitate precise registration of the sheet material with the print head.
Other objects and advantages of the present invention will become apparent in view of the following detailed description and accompanying drawings.